This invention relates to a communication system of a key telephone system wherein keys are used to select an office line and to hold the called line for calling an intercom line and more particularly to a communication system of a key telephone system wherein a key telephone or a station selected by the current supplied from key service unit is started to perform an individual call.
Such communication system of a key telephone has been well known in the art. For example, it is described in Chapter 3.1 Method of Calling Extension of the article entitled "Utilization of a Push Key Telephone" by G. Kita, A. Obata, et al. on pages 37 to 39 of IWATSU GIHO, Nov. 31, 1972, published by IWATSU. In such a system, a selection circuit for the key service unit is selected by the operation of a predetermined dial or key which is provided on the telephone constituting this system, for sending a call signal to a particular key telephone whereby the loudspeaker of the key telephone produces a voice signal or a call signal having a particular audible frequency. To respond to such call the called party must take up his handset. If the called party is sitting at a place remote from the handset he must walk to the handset. If the called party is absent when a call is made, the calling party must wait a long time.